This invention relates to the preparation of discrete, pourable, finely divided, tack-free plasticized filler particles and a method for the preparation thereof. More particularly, this invention relates to the use of such particles as fillers in the preparation of pourable, pulverulent rubber compounds having such characteristic properties.
The processing of rubbers is normally a multistage procedure. In contrast to the manufacture of molded components from thermoplastic synthetic resins, it is absolutely necessary, prior to the final shaping step, to conduct a generally complicated mixing process to obtain homogeneous incorporation of diverse additives prior to the cross-linking reaction. The high viscosity of the raw materials and their handling in bale form makes it necessary to utilize heavy machines, e.g. rolling mills or internal mixers, which perforce require a high energy consumption. The inherent considerable costs of such energy and equipment greatly impair the economy of the production of elastomeric materials.
Therefore, many efforts are now being directed toward adapting rubber technology to the principles of thermoplastic processing. An ideal starting point to attain this objective is rubbers in pulverulent form. Starting with the advantageous possibilities afforded in transportation and storage, the provision of automated feeding and conveying devices promises considerable savings in the feeding of mixing-and-deformation units. In order to incorporate additional ingredients, powder mixing plants can be utilized, which operate under very much more economical conditions. Thus, a continuous processing technique employing a high degree of automation has moved into the realm of possibility for the rubber industry.
In order to put the above-described ideas into practice, several methods which appeared suitable for the production of corresponding pulverulent rubber substances have been developed. Thus, German Unexamined Laid-Open Application DOS No. 2,135,266 and corresponding U.S. Pat. No. 3,846,365, the contents of which are incorporated by reference herein, disclose a process according to which discrete, finely divided, tack-free pourable elastomer particles are prepared from a filler-containing aqueous elastomer emulsion containing an aqueous emulsion of an elastomer dissolved in a volatile inert organic solvent having an emulsifying amount of quaternary ammonium salt cationic surfactant and an aqueous suspension or dispersion of a solid, finely divided elastomer filler, which process comprises gradually introducing the filler-containing aqueous elastomer emulsion into an aqueous alkali silicate solution while maintaining the solution at a pH greater than 2 and simultaneously evaporating the organic solvent to form discrete, finely divided, tack-free elastomer particles. The process can be applied to solid elastomers or to post-polymerization elastomer solutions, and is particularly suitable for preparing pourable carbon black-containing elastomer particles to be directly formed into shaped objects by molding, extrusion, etc.
It has furthermore been suggested in German Pat. Application No. P 22 14 121.5 and corresponding U.S. Pat. No. 3,895,035, the contents of which are incorporated by reference herein, to produce pourable, pulverulent rubber mixtures by emulsifying rubber solutions, optionally containing mineral oil plasticizers, in water in the presence of alkylamine oxyalkylates; introducing into these emulsions suspensions of solid fillers, preferably carbon blacks, optionally containing emulsifiers; continuously introducing the filler-containing, aqueous emulsions of the rubber solutions into hot aqueous alkali silicate solution, wherein the pH during the entire precipitation step ranges between 0.1 and 7, preferably between 1.0 and 3.5; simultaneously and continuously distilling off the rubber solvent; separating the thus-obtained, finely divided filler-containing rubber mixture from the water; and drying the product.
Another process for preparing pourable, filler-containing elastomer particles is described in U.S. Pat. application Ser. No. 421,819 filed Dec. 5, 1973, the contents of which are also incorporated by reference herein, wherein an admixture of a rubber solution and filler is flash-evaporated. As with the above and other processes, pourable, filler-containing elastomer particles amenable to thermoplastic processing techniques are obtained. Expecially valuable elastomers for use in such processes are the stereo block homopolymers of butadiene prepared according to the adiabatic polymerization process with lithium catalysts according to the techniques of U.S. Pat. No. 3,829,409, the contents of which are also incorporated by reference herein.
The primary objective of these processes is to obtain pourable, pulverulent rubber-filler mixtures wherein the customary homogenization of rubbers with fillers during the course of the processing operation is entirely eliminated. For certain important sectors of the rubber industry, especially tire manufacturing, it is advantageous if the rubber mixtures to be processed already contain the plasticizer provided as a further rubber constituent. In accordance with the above-described processes, the plasticizer can be immediately incorporated, to obtain pulverulent, pourable rubber mixtures. However, the pourable, pulverulent rubber-filler mixtures, containing plasticizer oil prepared according to this method exhibit a tendency toward adhesion with increasing additions of plasticizer oil, caused by the low-molecular weight hydrocarbon mineral oil plasticizer component. This leads to a clearly visible impairment of the pourability, as compared to otherwise identical rubber mixtures which are free of plasticizer oil. This phenomenon can become a very serious problem at high plasticizer oil dosages (40 parts by weight and thereabove, based on the rubber) during the handling of such pulverulent rubber mixtures. Thus, when dispensing the rubber mixtures from storage vessels or ensilage tanks, an irregular flow is observed, which can manifest itself in a surge-type discharge. This behavior is due, inter alia, to the fact that the pulverulent rubber mixtures containing plasticizer oil form a well during their discharge from the funnel-shaped vessels and then break off in chunks. Also, the notorious bridge formations within the storage tanks result in a nonuniform pouring of the rubber mixtures. These phenomena often lead to serious difficulties in the charging of processing machines.